Light olefins, defined herein as ethylene and propylene, individually or collectively, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Typically, the catalysts employed to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins in a hydrocarbon conversion apparatus (HCA) generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the oxygenate to olefin process incorporates a second step comprising catalyst regeneration. During regeneration, the coke contacts oxygen in a regeneration medium, typically air, under conditions effective to at least partially remove the coke from the catalyst by combustion thereby restoring the catalytic activity of the catalyst. The regenerated catalyst then may be reused to catalyze the conversion of oxygenates to olefins.
Typically, oxygenate to olefin conversion and regeneration are conducted in separate vessels. Coked catalyst used for conversion is continuously withdrawn from the HCA and directed to a catalyst regenerator, wherein the regeneration process occurs, and regenerated catalyst is continuously withdrawn from the catalyst regenerator and returned to the reaction HCA to facilitate conversion of oxygenates to light olefins.
Due to the typically substantial volume of coked catalyst processed by catalyst regenerators, catalyst regenerators may be very large vessels, oftentimes approaching the size of the HCA itself. Thus, the need also exists for reducing the size of catalyst regenerators associated with OTO reaction processes.